Lead zirconate titanate (PZT: Pb(ZrxTi1-x)O3, 0<x<1) is a typical ferroelectric material capable of storing a large amount of electric charge, and used in capacitors and film memories. PZT has pyroelectricity and piezoelectricity based on the ferroelectricity thereof. PZT has high piezoelectric performance, and its mechanical quality factor Qm can be controlled easily by adjusting the composition or adding an element thereto. This allows PZT to be applied to sensors, actuators, ultrasonic motors, filter circuits, and oscillators.
PZT, however, contains a large amount of lead. In recent years, there has been a growing concern that lead leached from waste may cause serious damage to the ecosystem and the environment. Accordingly, there has been an international movement toward restricting the use of lead. For this reason, non-lead-containing (that is, lead-free) ferroelectric materials, unlike PZT, have been in demand.
One of the lead-free ferroelectric materials that are currently under development is, for example, a perovskite-type composite oxide [(Bi0.5Na0.5)1-yBay]TiO3 made of bismuth (Bi), sodium (Na), barium (Ba), and titanium (Ti).
FIG. 11 shows the FIG. 1 in Patent Literature 1. The piezoelectric element shown in FIG. 11 comprises a substrate 11, an electrode layer 12, and a piezoelectric layer 13. These layers 11-13 are laminated in this order. An example of the material of the substrate 11 is zirconium oxide, aluminum oxide, magnesium oxide, nitride aluminum, or nitride silicon. An example of the material of the electrode layer 12 is platinum. An example of the material of the piezoelectric layer 13 is perovskite-like composite oxide [(Bi0.5Na0.5)1-yBay]TiO3.
The present inventors fabricated a piezoelectric film comprising a laminate of Si(100)/Pt(111)/perovskite-like composite oxide [(Bi0.5Na0.5)1-yBay]TiO3 according to Patent Literature 1 except that a Si substrate was used. However, the piezoelectric film was little oriented to a (001) orientation as described in the comparative example 1, which is described later.
Non Patent Literature 1 discloses that a LaNiO3 layer disposed on a substrate has a (001) orientation regardless of the orientation direction of the substrate.
The present inventors arranged a LaNiO3 layer between a Pt (111) layer and a perovskite-like composite oxide [(Bi0.5Na0.5)1-yBay]TiO3 layer on the basis of Non Patent Literature 1. In other words, the present inventors fabricated a piezoelectric film comprising a laminate composed of Si(100)/Pt(111)/(001)LaNiO3 layer/perovskite-like composite oxide [(Bi0.5Na0.5)1-yBay]TiO3. However, the piezoelectric film was not sufficiently oriented to a (001) orientation as described in the comparative example 2, which is described later.
Patent Literature 2 discloses (NaxBiy)TiO0.5x+1.5y+2—BaTiO3 layers having various composition of bismuth and sodium.
The present inventors varied the composition of bismuth and sodium on the basis of “BNT—08 7” disclosed in Table 1 of Patent literature 2. The BNT—08 7 is a (NaxBiy)TiO0.5x+1.5y+2—BaTiO3 layer (x=0.29, y=0.43). This allows the sufficient orientation to a (001) orientation. In other words, the present inventors fabricated a piezoelectric film comprising a laminate composed of Si(100)/Pt(111)/(001)LaNiO3 layer/perovskite-like composite oxide [(Bi0.43Na0.29)1-yBay]TiO3.
However, an amount of the deformation of the obtained piezoelectric film with regard to an applied voltage varied like a quadratic function as described in the comparative example 3, which is described later.
An amount of the deformation of a piezoelectric film with regard to an applied electric field is described below with reference to FIG. 12.
The amount of the deformation is required to be proportional to an electric field for an angular velocity sensor capable of measuring an exact angular velocity, an ink jet head capable of spraying an exact amount of ink, and a piezoelectric generating element capable of generating electric power due to positive piezoelectric effect. In other words, the amount B of the deformation and the electric field A are required to satisfy the following equation (1).B=c2·A  Equation (1)                (c1 is constant)        
The term “proportion” used in the present specification means that an amount B of the deformation and an electric field A satisfy the above-mentioned equation 1. In other words, the term “proportion” means a linear function. The term “proportion” does not include a quadratic function.
FIG. 12(a) shows a graph of an electric field—an amount of the deformation of the piezoelectric film where the amount of the deformation is proportional to the electric field. See FIG. 4 in Patent Literature 2. FIG. 12(b) shows an enlarged view of a portion surrounded by a dashed line in FIG. 12(a).
As shown in FIG. 12(b), the slope of a tangent line at the dot A is the substantially same as the slope of a tangent line at the dot B. The phrase “substantially same” means that a ratio represented by the slope of the tangent line at the dot A/the slope of the tangent line at the dot B is not less than 0.8 and not more than 1.2. This means that the amount B of the deformation is proportional to the electric field A. The electric field intensities at the dot A and at the dot B are, for example, 3 volts/micrometer and 10 volts/micrometer, respectively.
On the contrary, FIG. 12(c) shows a graph of an electric field—an amount of the deformation of the piezoelectric film where the amount of the deformation varies like a quadratic function with regard to the electric field. FIG. 12(d) shows an enlarged view of a portion surrounded by a dashed line in FIG. 12(c).
As shown in FIG. 12(d), the tangent line at the dot C has a shallower slope than the tangent line at the dot D. This means that the amount B of deformation varies with regard to the electric field A according to the following equation (2).B=c2A2  Equation 2                (c2 is constant)        
The dot C and the dot D have the same electric field intensities as the dot A and the dot B, respectively.
When the amount B of the deformation and the electric field A have a relationship of a quadratic function, it is difficult to measure an exact angular velocity, to produce an exact amount of ink, and to generate electric power due to positive piezoelectric effect. The relationship of a quadratic function between the amount b of the deformation and the electric field A is not suitable for an angular velocity sensor capable of measuring an exact angular velocity, an ink jet head capable of spraying an exact amount of ink, and a piezoelectric generating element capable of generating electric power due to positive piezoelectric effect.
However, as described in the comparative example 3, the amount B of the deformation of the obtained piezoelectric film varied as shown in FIG. 12(c) and FIG. 12(d). Accordingly, the piezoelectric film is not suitable for an angular velocity sensor, an ink jet head, and a piezoelectric generating element.